How to select the best ionic liquid
for a given application?
Koen Binnemans
Catholic University of Leuven (Belgium)
Cations of ionic liquids
R1
N
N
N
N+
R
R2
R
2
1
imidazolium
R4
R3
3
pyridinium
ammonium
O
R1
N+
R1
P+
+
N
R2
4
pyrrolidinium
R1
R2
R4
R3
R2
5
morpholinium
6
phosphonium
Anions of ionic liquids
•
•
•
•
•
•
•
•
•
•
•
[AlCl4]-, [Al2Cl7]Cl-, Br-, ISCN-, [N(CN)2][NO3]-, [SO4]2[CH3COO]-, [CF3COO][CF3SO3]- (= OTf)
[BF4][PF6]-, [SbF6][(CF3SO2)2N]- (= Tf2N)
[Co(CO)4]….. many more other anions
Number of possible ionic liquids?
•
By combination of cations, anions, chains lengths, substitution pattern,
chirality, …:
– About one million (106) simple ionic liquids
– About one billion (1012) binary ionic liquids
– About one trillion (1018) ternary ionic liquids
According to Ken Seddon (QUILL, Belfast)
•
But: until quite recently more than 50% of all publications were about
one class of ionic liquids!!! [Cnmim][PF6]
and in particular:
[C4mim][PF6]
[BMIM][PF6]
PF 6
N
N
Why was [BMIM][PF6] so popular?
•
•
•
Relatively cheap
Easy synthesis and purification
Excellent model compound for fundamental studies (high symmetry of
anion, weak hydrogen bonding)
• Hydrophobic: not miscible with water
• …. Everyone was using it ("me too" mentality)
…. The ionic liquid [BMIM][PF6 ] is a strong candidate for use as a recyclable solvent, the
Belfast team suggests. It is readily prepared at room temperature, is stable to moisture,
and has no detectable vapor pressure. Furthermore, it is immiscible with water and
hexane but dissolves many organic compounds and transition- metal complexes. And it
is a liquid from just above 0 °C to more than 200 °C. "This is my favorite roomtemperature ionic liquid since it is water stable and water immiscible," comments
Robin Rogers chemistry professor at the University of Alabama, Tuscaloosa. "The big
advantage I see here is product recovery and no solvent loss through evaporation.
Seddon's work furthers the idea that room-temperature ionic liquids can be used as
replacements for volatile organic compounds. I see an almost infinite variety of potential
reactions in these liquids."
(Chemistry and Engineering News, 4 Jan. 1999)
Things can change…
Why is [BMIM][PF6] problematic?
•
•
[PF6]- is hydrolytically unstable.
Contact with water above 50°C leads to HF formation
Every mole of [BMIM][PF6] produces 5 moles of HF!!
[PF6]- + 4H2O 5HF + F- + H3PO4
•
•
•
Measurements above 50°C lead to HF formation.
High viscosity (371 cP at 25 °C)
Because of its instability, [BMIM][PF6] will never be used as a solvent
by the chemical industry.
• [BMIM][PF6] is now considered as the « Antichrist » of ionic liquids
Is there still a future for hexafluorophosphate
ionic liquids?
•
[BMIM][PF6] must not be used as:
– solvent for chemical reactions
– organic phase for solvent extraction
•
But: LiPF6 is still widely used as electrolyte in Li-ion batteries
Hexafluorophosphate can be used in electrochemical devices
(anhydrous electrolytes)
• PF6- is a useful ion for the design of ionic liquid crystals
1-Alkyl-3-methylimidazolium salts, [Cnmim][X]
[Cnmim][PF6]: liquid-crystalline for C14 chain and longer
[Cnmim][Tf2N]: not liquid-crystalline
What is a good alternative for [BMIM][PF6] ?
•
Bistriflimide ionic liquid [BMIM][Tf2N]
(CF3SO2)2NN
•
N
Many names for one and the same anion:
– Bistriflimide
– Triflimide
– Bis(trifluoromethylsulfonyl)imide
– Bis(trifluoromethylsulfonyl)amide
– Bis(perfluoromethylsulfonyl)imide
– Bis(trifluoromethanesulfonyl)imide
(CF3SO2)2NPMS
Tf2NBTA
TFSI
Is [BMIM][Tf2N] the ideal ionic liquid?
•
[BMIM][Tf2N] is now often considered as the standard ionic liquid for
general use:
– Solvent for chemical reactions
– Organic phase in solvent extraction
– Electrolyte for electrodeposition of metals
• However.....
– Imidazolium not stable against strong bases (carbene formation)
– Limited cathodic stability (reduction)
– Limited stability against oxidation (ozonolysis)
– Many metal salts and biopolymers (cellulose) are poorly soluble in
ionic liquids with Tf2N- anions
Electrochemical windows of [Tf2N]- salts
Figure from Merck catalogue
Ozonolysis in ionic liquids
O
O3
HO
O
O
HO
hydroxy-ester
O
O
O3
O
O
+
H 2O
carboxy-ester
O3
+
MTP
O
H 3C
OH
O
lactone
MTP: 2-methoxytetrahydropyran
• Ozone = interesting alternative for less green oxidants
• Disadvantages ozone:
• explosive mixtures with organic solvents
• aerosol formation upon bubbling of ozone through solution
• Ionic liquids offer solution
• very low vapor pressure
• high viscosity
Ozonolysis in ionic liquids
•
Ozonation stability test
(0.42 mmol O3 per minute for 2 hours at 100 °C)
• Ozone resistance evaluated at the molecular level by 1H and 13C NMR
and macroscopically by colour and viscosity changes
• Fast degradation of imidazolium ionic liquids
sensitivity of double bonds to ozone
• Good ozone stability for pyrrolidinium salts
[BMPyr][(CN)2N]
and
[BMPyr][Tf2N]
Van Doorslaer et al., Chem. Commun. (2009) 6439
Selection criteria for ionic liquids
• Miscibility with water or organic solvents (temperature dependent)
• (Electro)chemical stability
• Solubility of solutes
• Melting point
• Viscosity
•
•
•
Some ionic liquids have wide applicability
– [BMIM][Tf2N] for organic synthesis
– [BMPyr][Tf2N] for electrodeposition of reactive metals
– [EMIM][OAc] for dissolution of cellulose
In other cases, series of ionic liquids have to be screened
Mixtures of ionic liquids can have unexpected properties
Hydrogenolysis of aromatic ketones to
alkylbenzenes
OH
O
R
H2
[Pd]
R
H+
R
[Pd]
H2
[Pd]
H2 O
Alkylbenzenes: intermediates for alkylbenzene sulfonate surfactants
Advantages of ionic liquids in catalytic hydrogenolysis
• improving recyclability Pd catalyst: immobilization of Pd catalyst in IL
• improved product separation by change in polarity
aromatic ketone (polar) → alkylbenzene (nonpolar)
• acidic solvent for promoting hydrogenolysis: IL with acidic functional group
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
R
Biphase catalysis in ionic liquids
Homogeneous phase
Substrate
reagentia
S
Heating
IL
Kat
Cat
S
IL
Cooling
P
Kat
Cat
producten
Product
P
Phase separation
Kat
Cat
IL
Reaction rate
Recyclage Cat
Recycling
Kat ++ IL
IL
25°C
S = substrate P = product
Cat = catalyst IL = ionic liquid
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Product must be less polar
than substrate
90°C
Miscibility of ionic liquids with
aromatic ketones and alkylbenzenes
•
Ideal ionic liquid:
– homogeneous mixture at reaction temperature (80 °C)
– phase separation after cooling to RT
(product isolation by decantation)
or
– miscible with aromatic ketone substrates at 80 °C
– immiscible with derived alkylbenzene products at RT
•
Reaction mixture composition at 70 % conversion was simulated with
a 30 mol% ketone/70 mol% alkylbenzene mixture (‘30/70’)
• Water was added to closely simulate reaction conditions : water
generated as by-product in alkylbenzene formation.
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Selected ketones and alkylbenzenes
O
C2
O
C4
O
C8
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Selected ionic liquids
• Screening of more than 30 different ionic liquids
• Cations:
– Methyltrioctylammonium (MOct3N)
– Choline (Chol)
– 1-Ethyl-3-methylimidazolium (EMIM)
– 1-Butyl-3-methylimidazolium(BMIM)
– 1-Butyl-2,3-dimethylimidazolium (BMMIM)
– N-butyl-N-methylpyrrolidinium (BMPyr)
– Different cations functionalized with COOH function
• Anions: Cl-, BF4-, Tf2N-, CF3COO-, CH3COO-, CH3SO3-, CF3SO3-,
Tosylate, EtSO4-, HSO4-, N(CN)2• No hexafluorophosphate (PF6) salts: hydrolyze with HF formation in
the presence of water at 80 °C
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Results of miscibility tests
• Limited number of tests successful.
• Methyltrioctylammonium salts: negative results.
• Imidazolium salts: positive results for [BMIM][Tf2N] and
[BMMIM][Tf2N], but only for tests with octanophenone.
• Pyrrolidinium salts: positive results for ([BMPyr][CF3CO2] in
combination with butyrophenone and for [BMPyr][(CN)2N] in
combination with all ketones.
• Good results for choline bistriflimide [Chol][Tf2N]
OH
N+
•
Tf2N-
Good results for most COOH-functionalized ionic liquids and
especiallyfor betainium bistriflimide [Hbet][Tf2N]
COOH
N+
Tf 2N-
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Catalytic experiments
1. Reactor
10 mL reactor
2 mL ionic liquid
50 bar H2
80 °C
2 hours
2. Analysis
Procedure
Upper layer reaction mixture (diluted in toluene) is injected in GC with FID and
apolar column (CB-sil 5)
Identification components
Injection pure components + GC-MS
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Product isolation
Product phase
IL + catalyst
Phase separation after hydrogenolysis of octanophenone
Product phase:
less than 0.01mol% IL leaching (NMR)
No measurable Pd leaching (ICP-AES)
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Mixtures of ionic liquids
100
[%] X
80
[%] S
%
60
40
20
0
[Chol][Tf2N]
[Hbet][Tf2N]
[Chol][Tf2N] +
[Hbet][Tf2N]
Reaction conditions: acetophenone (6 mmol), 0.08 g of 5 wt.% Pd-C,
[Chol][Tf2 N] (2 mL), [Hbet][Tf2 N] (1 mL), 80 °C, 50 bar H2 , 2 h; X [%] =
conversion of ketone; S [%] = selectivity towards ethylbenzene;
1+1≠2
Synergism in ionic liquid mixtures
Van Doorslaer et al., ChemSusChem 1 (2008) 997.
Design your own ionic liquid: Liquid metal salts
•
•
•
•
•
Need for high concentrations of metal ions in electrolytes for
electrodeposition
New approach: make metal part of ionic liquid
– No longer a need to dissolve metal salts
– High metal concentrations
Until now most metal-containing ILs had metal in anion
e.g. [AlCl4]- or [CoCl4]2these anions are not electro-active: not interesting for electrodeposition
Metal in cation is attracted toward cathode where electrodeposition
takes place.
Try to design low-melting metal salts (liquid metal salts) with the
metal an integral part of the cation.
Liquid metal salts
•
Tetrahedral metal ions with four ligands are analogues of
tetraalkylammonium or phosphonium ions
• For example, copper(I) with four neutral ligands
R
R
L
N+
P+
M n+
R
R
R
R
•
•
R
L
R
Choice of anion important for lowering the melting point
Metal preferably in lowest available oxidation state
Brooks et al., Chem. Eur. J. 17 (2011) 5054.
L
L
[Cu(MeCN)4][Tf2N]
•
Synthesis:
CuO + 2 HTf2N → Cu(Tf2N)2 + H2O
Cu(Tf2N)2 + Cu(s) + 8 CH3CN → 2 [Cu(CH3CN)4][Tf2N]
• Melting point: 66 °C
• For comparison:
– [NMe4][Tf2N]
133 °C
– [NEt4][Tf2N]
109 °C
– [NPr4][Tf2N]
105 °C
– [Cu(MeCN)4][OTf]
124 °C
Brooks et al., Chem. Eur. J. 17 (2011) 5054.
Electrochemistry of [Cu(MeCN)4][Tf2N]
•
Because cathodic decomposition reaction of ionic liquid is the
reduction of the metal ion to the metallic state, high current densities
can be obtained.
• Anodic reaction is stripping of copper from sacrificial electrode
Brooks et al., Chem. Eur. J. 17 (2011) 5054.
Deposit from [Cu(MeCN)4][Tf2N]
•
•
•
Current density: 25 A.dm-2
Deposit average thickness: 1 µm
Time for deposit: 5s
•
EDX analysis shows no decomposition products included in layer
Brooks et al., Chem. Eur. J. 17 (2011) 5054.
Copper(II)-containing Liquid Metal Salts
[Cu(MeIz)6][Tf2N]2
[Cu(BuIz)4][Cl]2
[Cu(BuIz)4][Tf2N]2
[Cu(BuIz)4][NO3]2
[Cu(HexIz)4][Cl]2
[Cu(MeIz)6][Tf2N]2
[Cu(1,2-DiMeIz)4][BF4]2
50 °C
55 °C
< RT
126 °C
66 °C
50 °C
185 °C
N
N
N
N
N
N
N
N
N
N
R
R = Bu,Hex
MeIz, BuIz, HexIz
1,2-DiMeIz
2-MeIz
4-MeIz
Liquid Metal Salts of other transition metals
[Ni(2-MeIz)4Cl][Cl]
[Ni(BuIz)6][Cl] 2
[Ni(HexIz)6][Cl] 2
[Co(HexIz)6][Cl] 2
[Fe(MeIz)6][NO3] 2
[Mn(BuIz)6][Cl] 2
[Mn(HexIz) 6][Cl] 2
83 °C
124 °C
69 °C
34 °C
60 °C
58 °C
45 °C
[Ni(2-MeIz)4Cl][Cl]
N
N
N
N
N
N
N
N
N
N
R
R = Bu,Hex
MeIz, BuIz, HexIz
1,2-DiMeIz
2-MeIz
4-MeIz
83 °C
Conclusions
•
•
•
•
•
•
•
•
[BMIM][Tf2N] is preferred over [BMIM][PF6] as a solvent for organic
reactions and extraction studies
There can be stability issues with imidazolium ILs
– Stability in alkaline medium
– Stability against oxidation and reduction
Pyrrolidinium ionic liquids have higher electrochemical stability
Screening of ionic liquids can be required to find best system
(screening sets are commercially available)
Mixtures often perform better than pure Ils
Biopolymers are not soluble in Tf2N- ILs
Inorganic salts are often poorly soluble in Tf2N- ILs
Liquid metal salts have high metal concentration
Acknowledgments
•
K.U.Leuven
– Dr. Peter Nockemann (now at QUILL, Belfast)
– Prof. Dr. Rik Van Deun (now at UGhent, Belgium)
– Dr. Ben Thijs
– Prof. Tatjana Parac-Vogt, Dr. Kris Driesen
– Dr. Neil Brooks, Evert Vanecht, Hasan Mehdi, Lethesh K.C., Dr. Kyra Lunstroot
– Prof. Dr. Luc Van Meervelt, Dr. Kristof Van Hecke
– Dr. Neil Brooks
– Prof. Dr. Dirk De Vos, Prof. Dr. Jan Fransaer, Prof. Dr. Christ Glorieux
– Stijn Schaltin, Charlie Van Doorslaer, Igor Ignatiev, Dr. Jan Leys, Sil Wellens
Sandra Hojniak, Justyna Szytniewska
•
Universität Leipzig (Germany)
– Stefan Zahn, Prof. Barbara Kirchner
•
Université Montpellier (France)
– Prof. André Vioux, Prof. Jean Le Bideau,
Dr. Séverine Bellayer, Dr. Lydie Viau
Financial support by K.U.Leuven, FWO-Vlaanderen, IWT
Thank you!
http://www.kuleuven.be/ionic-liquids/